Compound Interest Formula
Compound interest - meaning that the interest you earn each year is added to your principal, so that the balance doesn't merely grow, it grows at an increasing rate - is one of the most useful concepts in finance. It is the basis of everything from a personal sings plan to the long term growth of the stock market. It also accounts for the effects of inflation, and the importance of paying down your debt.
See How Finance Works for the compound interest formula, (or the advanced formula with annual additions), as well as a calculator for periodic and continuous compounding.
If you'd like to know how to estimate compound interest, see the article on The Rule of 72.
(Also compare simple interest.)